Network route appear in the routing table as “O” for all routers on area and ” C “ for direclty connected ( check reference 2) O mean OSPF .

This LSA will be flooded to all the routers on the area & it ends at area boundaries.

Flooding mean that any router on the area will have all routers Type-1LSA’s for that area. the way this done is that every router in the area will send every type-1 LSA it receive to other routers. sequence number are used to prevent duplication.

Each router creates only 1 LSA that contains all links information under that OSPF process (any update of that LSA will include all that router links),

LSA changes will make all routes re-run the full SPF tree ( you will see the sequence number change )

Contain the router links information and as you can see on the photo we have point-to-point and broadcast lines.

The way that a link information is described dependeds on the link type :1 : Link connected to broadcast or NBMA network but with out neighbors.

described as Stub network

2 : Link connected to broadcast or NBMA network with neighbors.

described as Transit network

3 : Link connected to Point-to-Point

described twice : as Point-to-Point network & as Stub network (both at the same time)

Command output :
this output is taken from Router 4 and when going to (reference 1) you will see (3 under the links ) so where is the third link, since we know that R4 has only 2 connections. one of them point-to-point.

This summary route informs routers in an area about networks existence on other areas .

R5 will create summary routes for 3 areas,

one for area 1 to tell that area about networks in area 0 & area 2 (he knows about area 2 networks from the type 3 advertised by R10) ,

one for area 0 to tell it about networks in area 1,,

R10 and any other ABR will do the same job with the areas connect to it.

Change on LSA’s will not recalculate the full SPF tree line type 1 & 2 , partial SPF will only run for that network, because unlike Type 1&2 there is a different LSA for each network with it’s own sequence number, so a partial SPF will run for this network (or this LSA which include 1 network) only .

The ABR creates a separate LSA for each network, so it doesn’t need to resend all networks again when ever one of them goes down it will only change that network LSA.Command output :

type-2 metric : the metric will reflect the ASBRs advertised cost only (E2) which is 20 by default.

the forward-address is used to avoid extra hops (avoid sub-optimal routing)

i am going to create a separate topic on that matter but for now you may want to know that forward-address is set to non-zero for network 200 because of the NSSA area, i will explain why in another topic.

Type 4 mertric also help with choosing the best path to the ASBR when the forward address is set to 0.0.0.0 . ( imagine 2 ABR’s both saying we can reach the ASBR but each of them have a different metric )

now let’s take a step back to type 5,

as you can see that R10 is the advertiser for network 200.

why not R11 ? and why R10 is advertised as AS Boundary Router address (ASBR ) while the ASBR should connect OSPF with non-ospf ? Because R10 translated Type 7 to type 5 , so R10 is ABR & ASBR at the same time.

Type 7 (nssa-external)

created by the NSSA ASBR & it doesn’t leave the nssa area boundaries .

like type 5 it has metric type-1 (N1) or type-2 metric (N2) . N1 will be be translated to E1 and N2 to E2

the ABR will convert the type 7 to type 5 and that type 5 will be flooded to all the OSPF domain .and that’s what makes that ABR to be ASBR as well . because it’s creating type 5 .

type-4 will point to the ASBR which created the type 5 (R 10), but the forward-address inside type 5 will point to the actual ASBR which created type 7 ( R11 ) .